This invention relates in general to overvoltage protectors, and more specifically, to an overvoltage protector bridge circuit for protecting against excess voltage in high frequency applications.
Overvoltage protectors are used with telephone lines to protect telecommunications equipment against excess voltage occurring on the outside lines, such as from lightning strikes or induction from power lines. The telephone company incoming lines are made up of a large number of twisted pairs of wires, one of the wires of each pair being referred to as a tip wire and the other a ring wire. A xe2x80x9ccrowbarxe2x80x9d type overvoltage protector for each of the twisted pairs will connect to ground. In the event of excess voltage between the tip and ring wires, or between both the tip and ring wires and ground, the overvoltage protector will conduct, thereby creating a short to ground and diverting excess voltages and currents away from equipment and personnel.
There are different types of overvoltage protectors for high frequency applications. The two primary categories of overvoltage protectors for high frequency applications are: a clamping device, which holds the voltage to a specific threshold; and a crowbar device that shorts to ground when an overvoltage event occurs. In preventing damage from excess voltage, present practice allows for placement of overvoltage protectors that increase the ability to handle sustained or transient current. However, an overvoltage protector capable of achieving this goal often has a high off-state capacitance that causes attenuation and reflection at high frequencies, and thereby interferes with signal transmission. As presently designed, solid state overvoltage protectors having a lower off-state capacitance concomitantly have a decreased ability to handle excessive voltages.
One type of overvoltage protector presently in use is a gas tube. Electrical contacts are separated by a gap within a hermetically sealed chamber that is evacuated of air and filed with an inert gas. While gas tube overvoltage protectors work well at high frequencies due to very low off-state capacitance, repeated voltage surges will produce sputtering on the contacts. As a result, the protector will eventually require replacement.
Another type of overvoltage protector is a solid state overvoltage protector, such as a thyristor or metal oxide varistor (MOV). When a voltage above a preselected minimum is encountered, the solid state overvoltage protector conducts to ground. Solid state overvoltage protectors work well for moderate current loads and moderate frequencies. Furthermore, solid state overvoltage protectors, unlike gas tube protectors, are capable of repeatedly conducting to ground without incurring substantial damage. Solid state overvoltage protectors have also been used in bridge circuits that include diodes to steer the excess voltage. The diodes steer the excess voltage through the solid state overvoltage protector in the same direction regardless of whether the excess voltage is on the tip line relative to the ring line, on the ring line relative to the tip line, or on both the tip and the ring lines relative to ground.
Telephone lines are increasingly being used to transmit high frequency signals. Frequencies of 30 kilohertz to 30 megahertz are typically employed over telephone lines for data transmission. Solid state overvoltage protectors, and in particular thyristors and MOVs, have a fairly high capacitance, for example from about 70 to about 200 picofarads (pf). This high capacitance creates a problem with high frequency signals. High off-state capacitance will cause capacitive impedance mismatches on the twisted pair transmission line, which typically will result in high signal attenuation and reflectance (i.e., return loss) at high frequencies.
Present bridge circuit designs have legs connecting the tip and ring lines. Protection in such circuits is achieved by virtue of the diodes placed along the pathways guiding the excess a i voltage through a solid state overvoltage protector to ground. Thus, the diodes must be selected to carry the full overvoltage event or parallel diodes must be selected to share the full overvoltage event. Furthermore, diodes along the pathways to ground must be able to carry double the current of a single overvoltage event, as events occurring simultaneously on both the tip and ring conductor will use only one of those paths to ground. The implication of this diode selection limitation is that a diode to be placed on a leg of a bridge circuit having sufficient current handling capability will exceed the off-state capacitance requirements for high frequency applications.
In this invention, an overvoltage protector bridge circuit is provided in which the overall off-state capacitance is reduced to maintain the integrity of high-speed data transmission while preserving current handling capability. The bridge circuit incorporates multiple diodes in series or in parallel along each of the legs of the circuit, thereby reducing off-state capacitance while retaining the surge current handling capability of each leg. Placing multiple diodes in series along each leg of the bridge circuit reduces the off-state capacitance. Placing multiple diodes in parallel along each leg of the bridge circuit increases the surge current handling capability of each leg. Multiple configurations may be chosen from diodes of varying capacitance to optimize the protective characteristics of a particular bridge circuit while adding the capability of operating at high frequency with minimal attenuation and reflection. For example, multiple diodes in series may be placed along one or more legs of the bridge circuit while multiple diodes in parallel are placed along the remaining legs of the bridge circuit.